Radiation therapy and radiosurgery are established methods of treating patients with certain malignant and benign diseases. Radiation therapy, which is also referred to as radiotherapy, radiation oncology, and XRT, involves the administration of ionizing radiation to a patient, such as a patient being treated for cancer. Exposure of malignant cells to radiation inhibits their proliferation and induces cell death. A patient undergoing radiation therapy for cancer treatment is typically treated with ionizing radiation many times, often at defined intervals. A disadvantage of radiation therapy is that normal cells and tissues may be adversely affected by the radiation. Assuming that normal tissue recovers at a faster rate than cancerous tissue, administration of radiation at defined intervals is expected to allow any normal tissue, which has been adversely affected by the radiation, to recover before the next treatment. Radiation therapy also may be combined with surgery, chemotherapy, and/or hormone therapy. Radiation therapy is also used to treat various non-malignant conditions, such as trigeminal neuralgia, severe thyroid eye disease, pterygium, and pigmented villonodular synovitis, and to inhibit keloid scar growth and heterotopic ossification. However, the use of radiation therapy to treat non-malignant conditions is limited by the risk of radiation-induced cancer.
Multileaf collimators have been used with linear accelerators (LINAC) to improve the geometric conformation of the radiation treatment to the target tissue as a way of minimizing adverse effects on normal tissue. Multileaf collimators have many rectangular vanes or “leaves” of a material having a high atomic number, such as tungsten, that can be moved independently in and out of the path of a particle beam. When a “leaf” is in the path of a particle beam, the beam is blocked. Therefore, by positioning the “leaves” to form an aperture having a specific geometry, it is possible to shape a beam of radiation to conform to a desired geometric shape. Intensity modulated radiation therapy (IMRT) further improves the conformation by using multiple weighted apertures for each treatment beam to account for geometric variations in the dimensions of the beam projection.
Radiosurgery is a medical procedure that allows non-invasive treatment of benign and malignant tumors and other conditions, such as arteriovenous malformations (AVMs) and trigeminal neuralgia. It involves precisely directing highly focused beams of ionizing radiation at a single point (referred to as an “isocenter”). By applying a precise dosage of radiation to the isocenter, tumors and other lesions, which are otherwise inaccessible or inadequate for open surgery, can be ablated. Typically, only a single or a few treatments are necessary. Radiosurgery typically involves the use of multiple small circular collimated radiation beams directed at the isocenter. Consequently, radiation is concentrated at the isocenter through the superposition of multiple small overlapping beams. Adjacent tissues, which are not in the isocenter, are not exposed to the overlapping beams and, therefore, are not subjected to the concentrated dose of radiation.
The multiple overlapping beams may be created using many individual sources of radiation with individual collimators, such as in the case of the GammaKnife™ from Elekta (Stockholm, Sweden), or by arcing a single source and collimator about a point. Traditional isocentrically mounted medical LINACs achieve a similar effect of multiple independent beams using a combination of table and gantry angles with circular or multileaf collimators.
A complex tissue volume can be treated with radiosurgery using multiple isocenters (or “shots”) at discrete points inside the volume. Radiation is delivered to each isocenter with a specific arrangement of multiple beams and a specific collimator size.
The dose of radiation used in radiation therapy and radiosurgery is limited by concerns over toxicity to non-target tissue, i.e., normal tissue, within and/or adjacent to the target tissue, e.g., diseased tissue. The amount of radiation to which non-target tissue is exposed is determined by a number of factors. For example, there is a margin of error associated with the placement of the radiation beam, which is limited by the mechanical accuracy of the treatment device and the knowledge of where the non-target tissue is at the time of treatment. The nature and properties of a single divergent beam of radiation incident on the target tissue also impact the exposure of non-target tissue to radiation. The lateral range of secondary electrons outside the field of radiation, the physical size and shape of the source of the photons, the shape of the collimator edge, photon scatter, and transmission through the collimator affect the fall-off in dosage (i.e., the penumbra of the radiation beam) of high-energy photon radiation away from the edge of the collimator. The interaction of all beams also impacts the exposure of non-target tissue to radiation. Geometrically, all beams overlap in the target tissue, and they also may overlap in the non-target tissue. Such overlap in the target tissue and the non-target tissue makes it very difficult, if not impossible, to deliver a uniformly high dose of radiation to the target tissue while not delivering any radiation to the non-target tissue. Away from the edge of the target tissue, e.g., the tumor, the dose of radiation will also fall off with a varying gradient rather than abruptly. Depending on the techniques used, including the collimator design and the beam arrangement, such dose fall-off can be very steep or very gentle. Such dose fall-off at the target boundary is referred to as the “dose gradient.” The best therapeutic gain can be achieved with a high dose gradient at the edge of the target tissue. For a single divergent high-energy photon beam of radiation covering the target tissue, the “dulling” of dose gradient is caused by scattered photons and electrons along the radial dimension of the beam boundary and the beam's entrance and exit along the beam direction. With multiple beams, the dose gradient is largely influenced by the overlap of beams on the entry and exit sides of the target tissue. The divergence of radiation beams also contributes to making the dose gradient less steep. The use of multiple focal spots (also referred to as “shots”), such as when a volume of target tissue cannot be treated with a single isocenter, results in interaction of beams from adjacent focal spots. Interaction of beams from adjacent focal spots can increase the exposure of non-target tissue within and/or adjacent to the target tissue to radiation.
The main reason why the above methods result in exposure of non-target tissue to radiation is that the methods cannot create a sharp dose gradient at the boundary of a target tissue. The collimating systems are designed to treat targets of all sizes and shapes. With medical LINACs, the collimators are either fixed or dynamically configurable. Fixed collimators are invariably circular in shape. Dynamically configurable collimators are either composed of four jaws, which can shape any rectangular field, or use multiple leaves of varying thickness, which can shape irregular fields. Because target tissues, such as tumors, are rarely rectangular, the jaws have gradually become obsolete. For multileaf collimators, the beam boundary cannot be very sharp no matter how thin the leaves are. For ablative treatments, circular collimators are typically used. A circular collimator, however, cannot create a sharp dose fall-off between a target tissue and an adjacent non-target tissue when beams are overlapped to form an edge. This is illustrated in FIG. 2a and FIG. 2c. FIG. 2a is a schematic drawing of the movement of a cylindrical radiation beam along a linear interface (see “Brief Description of the Figures” for details). FIG. 2c is a graph of the cumulative radiation intensity (“I”) versus the location along the central axis of overlap between the adjacent cylindrical radiation beams (“y”) of FIG. 2a (see “Brief Description of the Figures” for details). As can be seen in FIG. 2c, the dose fall-off from the central axes and the outer edges of the cylindrical radiation beam along the central axis of overlap between adjacent cylindrical radiation beams is from about 95% to about 5%, which is on the order of the radius of the collimated beam. In other words, a circular collimator makes a “dull” knife for sculpting. External beam radiation therapy typically achieves a dose fall-off of about 90% to about 10% over about 10 mm, whereas radiosurgery with an ablative system typically achieves a dose fall-off of about 90% to about 10% over about 5 mm. Fixed collimators are advantageous in that they have small beam penumbra and precise divergence.
Moving a circular beam along a line is a special case. In general, beams are overlapped to shape curved surfaces in two dimensions as illustrated in FIG. 3a and FIG. 3c. FIG. 3a is a schematic drawing of the movement of a cylindrical radiation beam along a convex interface (see “Brief Description of the Figures” for details). FIG. 3c is a graph of the cumulative radiation intensity (“I”) versus the location along the radial dimension (“r”) of FIG. 3a (see “Brief Description of the Figures” for details). As can be seen in FIG. 3c, a circular collimator cannot form a completely “cold” (low dose) hole surrounded by “hot” (high dose) regions.
In addition to the above, a patient is held stationary on a patient support system, such as a table, during radiation treatment, and the radiation beam is moved by moving the collimator. The radiation beam can be rotated around the patient with a single isocenter. There is no dynamic coordination of the patient support system with the dynamic delivery of the radiation beam. The only known exception is the management of breathing-induced tumor motion by moving the patient support system exactly opposite that of the tumor in order to keep the tumor location stationary in space (D'Souza et al., “Intra-Fraction Motion Synchronized Adaptive Couch-Based Radiation Delivery: A Feasibility Study,” Phys. Med. Biol. 50: 4021-4033 (2005)).
Radiation therapy using external beams as described above typically places the tumor at the isocenter, i.e., the intersection of all rotational axes. This arrangement makes patient set-up much easier. However, in any beam direction, radiation can only be directed at a point in the target tissue through one unique path. This significantly limits how the target tissue is irradiated.
Some degree of freedom is possible with CyberKnife (Accuray Inc., Sunnyvale, Calif.). With CyberKnife, an x-band LINAC is mounted on a 6-axes robot, and the patient is kept stationary on a patient support system. Using circular collimators, CyberKnife delivers radiation doses to the target tissue by crossing many (e.g., a hundred or more), variously oriented, cylindrical radiation beams, which do not share a common center of rotation and, therefore, are not isocentric, in the target. While variously angled cylindrical radiation beams can be realized with the CyberKnife, configuring the beams can be complex and, since it involves the use of a robot, expensive. Furthermore, CyberKnife can only use a single source of radiation, which requires more time to treat a patient, and, since CyberKnife uses circular collimators, it cannot create a sharp dose fall-off between a target tissue and an adjacent non-target tissue when beams are overlapped to form an edge.
The present disclosure seeks to overcome the disadvantages inherent in currently available methods of radiation therapy. In view of this, it is an object of the present disclosure to provide a method, a collimator, and a system for dynamically sculpting target tissue so that the radiation doses at the boundaries of the target tissue and any non-target tissue(s) have a sharp fall-off, thereby reducing, if not eliminating, “dose spillage” to non-target tissue and delivering a uniform dose to the target tissue. Compared to currently available methods of radiation therapy, the method of the present disclosure is easier to use and more economical. It is another object of the present disclosure to provide a method of planning irradiation of a target tissue in accordance with the present disclosure. These and other objects and advantages, as well as inventive features, will become apparent from the detailed description provided herein.